A lithium secondary battery with a high energy density is expected to be used in various applications, not only for small IT equipment such as mobile phones and notebook PCs, but also as a medium and large-size battery for electric cars and power storage. In particular, there is a demand for development of a cathode material with a high degree of safety and a high energy density required for medium and large-sized lithium secondary batteries for electric vehicles and power storage. Generally, safe electrode materials with an excellent capacity based on a LiCoO2-based material, for example, LiMn2O4 (LMO) and a high-capacity LiMn1/3Co1/3Ni1/3O2 (NMC), have been studied for a lithium secondary battery. However, such materials basically have a low capacity limited a sufficient level of safety, and thus safe, excellent, and high-energy density materials need to be developed for common use of medium and large-sized batteries.
For electric cars, a distance over which a vehicle may travel on a single charge is crucial, which is related to an energy density of secondary battery cathode materials and thus, research and development on high-performance cathode materials is essential. Existing LMO or NMC and olivine cathode materials have an energy density of about 120 to 150 milliampere hours/gram (mAh/g), which is insufficient for a remarkable improvement in a driving distance of electric cars.
A Li2MnO3-based composite material basically has a specific high capacity of about 460 mAh/g and provides a high initial capacity of about 200 to 250 mAh/g in practice, with a relatively high average discharging voltage of about 3.5 volts (V). Thus, the Li2MnO3-based composite material is known as a next-generation cathode material candidate to realize a high capacity and a high energy density, and accordingly studies are being conducted on a high-efficiency method of synthesizing cathode materials with great potential for high performance.
Primary requirements for medium and large-sized lithium secondary batteries for electric cars and power storage are safety and a high energy density. Accordingly, research and development is conducted to secure safety for medium and large-sized lithium secondary batteries using conventional methods, such as a process of mixing a spinel-manganese LMO material with a comparatively high-capacity NMC material in a proper composition or a process of manufacturing an olivine-based LiFePO4 material which has a relatively low discharging voltage of about 3.0 V but provides a high degree of safety and a high capacity.
However, these conventional LMO, NMC, and olivine-based LiFePO4 materials provide low-capacity batteries and thus, have limitations in terms of improving a distance over which a vehicle may travel on a single charge.
Conventional cathode materials have a basic energy density of about 120 to 140 mAh/g, which is insufficient and thus, have limitations in realizing common use of applications needing high energy density, such as electric cars. Particularly, since iron phosphate materials receiving attention in recent years have a low voltage and definite limitations in capacity increase (4V, 150 mAh/g), developing cathode materials with an excellent energy density is a pressing concern. Batteries of conventional electrode materials mostly operate in a charging voltage range of 2.0 to 4.2 V.
Conventional cathode materials have issues in view of safety, cost, and energy density. A nickel-based LiNiO2 (LNO) material with excellent capacity does not provide a sufficient level of safety, a manganese-based LMO is insufficient in terms of capacity and durability, an NMC exhibits insufficient performance in terms of safety and cost, and iron phosphate materials are inadequate in terms of energy density and cost.
The conventional manganese-based materials with excellent safety have a low capacity and are not given significant consideration in terms of durability, and thus active research and development is not being conducted on these materials. Iron phosphate materials are also involved in studies on nanotechnology to obtain high-capacity electrode performance, resulting in an increased cost for nanoscale materials. Thus, since charging and discharging are performed in a range of about 2.0 to 4.2 V to secure safety of the convention materials, these materials basically have a limited discharging capacity.